Magnesium borohydride and its derivatives as magnesium ion transfer media

ABSTRACT

An anhydrous electrolyte for a magnesium battery. The anhydrous electrolyte includes a magnesium salt having the formula Mg(BH 4 ) 2 . The electrolyte also includes a solvent, the magnesium salt dissociating in the solvent. Various solvents including aprotic solvents and molten salts such as ionic liquids may be utilized. The magnesium salt dissociates in the solvent to Mg electroactive species BH 4   −  and Mg 2+ .

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/839,003 filed Mar. 15, 2013, which in turn is a continuation-in-partof Ser. No. 13/720,522 filed Dec. 19, 2012, which in turn claims thebenefit of U.S. Provisional Patent Application Ser. No. 61/678,672,filed Aug. 2, 2012, the entire contents of all are incorporated hereinby reference.

FIELD OF THE INVENTION

The invention relates to electrolytes and more particularly toelectrolytes for magnesium batteries.

BACKGROUND OF THE INVENTION

Rechargeable batteries, such as lithium-ion batteries, have numerouscommercial applications. Capacity density is an importantcharacteristic, and higher capacity densities are desirable for avariety of applications.

A magnesium ion in a magnesium or magnesium-ion battery carries twoelectrical charges, in contrast to the single charge of a lithium ion.Improved electrolyte materials would be very useful in order to develophigh capacity density batteries.

Current state of the art electrolytes for magnesium batteries may useorganomagnesium salts and complexes as they are the only ones known tobe compatible with an Mg anode allowing for reversible electrochemicalMg deposition and stripping. However, such materials may be corrosiveand may be difficult to utilize in a battery. Conventional inorganic andionic salts such as Mg(ClO₄)₂ may be incompatible with the Mg anode dueto the formation of an ion-blocking layer formed by theirelectrochemical reduction.

There is therefore a need in the art for an improved electrolyte thatsolves the problems of the prior art and provides a stable rechargeableMg battery system. There is a further need in the art for an electrolytethat allows reversible Mg deposition and stripping in a chloride-freeinorganic salt. There is also a need in the art for an improved batteryhaving increased current densities and high coulombic efficiencies.

SUMMARY OF THE INVENTION

In one aspect, there is disclosed an anhydrous electrolyte for amagnesium battery. The electrolyte includes a magnesium salt having theformula Mg(BH₄)₂. The electrolyte also includes a solvent, the magnesiumsalt dissociating in the solvent. Various solvents including aproticsolvents and molten salts such as ionic liquids may be utilized. Themagnesium salt dissociates in the solvent to Mg electroactive speciesBH₄ ⁻ and Mg²⁺.

In another aspect, there is disclosed a magnesium battery that includesa magnesium metal containing anode. The battery also includes anelectrolyte including a magnesium salt having the formula: Mg(BH₄)₂. Theelectrolyte also includes a solvent, the magnesium salt dissociating inthe solvent. Various solvents including aprotic solvents and moltensalts such as ionic liquids may be utilized. The magnesium saltdissociates in the solvent to Mg electroactive species BH₄ ⁻ and Mg²⁺.The battery also includes a cathode separated from the anode. Magnesiumcations are reversibly stripped and deposited between the anode andcathode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of 0.5 M Mg(BH₄)₂/THF showing a) Cyclic voltammetry(8 cycles) with the inset showing deposition/stripping charge balance(3^(rd) cycle) b) XRD results following galvanostatic deposition of Mgon a Pt working electrode, c) Cyclic voltammetry for 0.1 M Mg(BH₄)₂/DMEcompared to 0.5 M Mg(BH₄)₂/THF with the inset showingdeposition/stripping charge balance for Mg(BH₄)₂/DME;

FIG. 2 is a diagram of Mg(BH₄)₂ in THF and DME: a) IR Spectra, b) ¹¹BNMR, and c) ¹H NMR;

FIG. 3 is a diagram of LiBH₄ (0.6 M)/Mg(BH₄)₂ (0.18 M) in DME: a) cyclicvoltammetry with the inset showing deposition/stripping charge balance,b) XRD results following galvanostatic deposition of Mg on a Pt disk andc) IR spectra (I indicates band maxima for Mg(BH₄)₂/DME);

FIG. 4 is a diagram of Charge/discharge profiles with Mg anode/Chevrelphase cathode for 3.3:1 molar LiBH₄/Mg(BH₄)₂ in DME;

FIG. 5 is a NMR scan of a compound of the formula MgB_(x)H_(y), wherex=11-12 and y=11-12;

FIG. 6 is a plot of the Current density as a function of Potential for acompound of the formula MgB_(x)H_(y), where x=11-12 and y=11-12.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

There is disclosed a novel electrolyte for an Mg battery. The novelelectrolyte allows electrochemical reversible Mg deposition andstripping in a halide-free inorganic salt.

In a first aspect the novel electrolyte may include a magnesium salthaving the formula Mg(BX₄)₂ where X is selected from H, F and O-alkyl.The magnesium salt is dissolved in the solvent. Various solventsincluding aprotic solvents and molten salts such as ionic liquids may beutilized. Aprotic solvents may include, for example solvents such astetrahydrofuran (THF) and dimethoxyethane (DME). Other examples ofaprotic solvents include: dioxane, triethyl amine, diisopropyl ether,diethyl ether, t-butyl methyl ether (MTBE), 1,2-dimethoxyethane (glyme),2-methoxyethyl ether (diglyme), tetraglyme, and polyethylene glycoldimethyl ether.

In one aspect, the magnesium salt may have a molarity of from 0.01 to 4molar.

The electrolyte may further include a chelating agent. Various chelatingagents including glymes and crown ethers may be utilized. The chelatingagent may be included to increase the current and lower theover-potential of a battery that includes the electrolyte.

The electrolyte may further include acidic cation additives increasingthe current density and providing a high coulombic efficiency. Examplesof acidic cation additives include lithium borohydride, sodiumborohydride and potassium borohydride. The acidic cation additives maybe present in an amount of up to five times the amount in relation toMg(BX₄)₂.

In another aspect the novel electrolyte may include a magnesium salthaving the formula a magnesium salt having the formulaMg(B_(X)H_(y))_(z) where X=3-12, y=8-12 and z=1-2. The magnesium salt isdissolved in the solvent. Various solvents including aprotic solventsand molten salts such as ionic liquids may be utilized. Aprotic solventsmay include, for example solvents such as tetrahydrofuran (THF) anddimethoxyethane (DME). In one aspect, the magnesium salt may have amolarity of from 0.01 to 4 molar.

The electrolyte may further include a chelating agent. Various chelatingagents including monoglyme may be utilized. The chelating agent may beincluded to increase the current and lower the over-potential of abattery that includes the electrolyte.

The electrolyte may further include acidic cation additives increasingthe current density and providing a high coulombic efficiency. Examplesof acidic cation additives include lithium borohydride, sodiumborohydride and potassium borohydride. The acidic cation additives maybe present in an amount of up to five times the amount in relation toMg(B_(x)H_(y))_(z).

In a further aspect, there is disclosed a magnesium battery thatincludes a magnesium metal containing anode, an electrolyte of eitherthe formula: Mg(B_(x)H_(y))_(z) where X=3-12, y=8-12 and z=1-2 orMg(BX₄)₂ where X is selected from H, F and O-alkyl and a cathode. Theelectrolyte may also include the chelating agents and acidic cationadditives as described above.

The anode may include magnesium metal anodes. The cathode may includevarious materials that show an electrochemical reaction at a higherelectrode potential than the anode. Examples of cathode materialsinclude transition metal oxides, sulfides, fluorides, chlorides orsulphur and Chevrel phase materials such as Mo₆S₈. The battery includesmagnesium cations that are reversibly stripped and deposited between theanode and cathode.

In one aspect, magnesium boron based compounds having the formulaMgB_(x)H_(y), where x=11-12 and y=11-12 may be utilized to provide animproved stability towards electrochemical oxidation so thatelectrolytes may be utilized with high voltage cathodes such as Mn02. Inanother aspect, a mixture of compounds having the formula MgB_(x)H_(y),where x=11-12 and y=11-12 may be utilized.

Prior art attempts to synthesize MgB₁₂H₁₂ were conducted in aqueousmedia and have resulted in the formation of MgB₁₂H₁₂.H20 with waterstrongly coordinated to the compound. Attempts to remove H20 have beenproblematic as outlined in the prior art. For example, (Chen, X.;Lingam, H. K.; Huang, Z.; Yisgedu, T.; Zhao, J.-C.; Shore, S. G. ThermalDecomposition Behavior of Hydrated Magnesium Dodecahydrododecaborates.J. Phys. Chem. Lett. 2010, 1, 201-204) documented the difficulty inremoving H2O from the compounds.

For use as an electrolyte in an Mg battery, compounds of the formulaMgB_(x)H_(y), where x=11-12 and y=11-12 should be free of H2O or water.In one aspect, MgB₁₂H₁₂, that is water free may be synthesized inaprotic media that results in the formation of water free compounds ofthe formula MgB_(x)H_(y), where x=11-12 and y=11-12.

Examples

Magnesium borohydride (Mg(BH₄)₂, 95%), lithium borohydride (LiBH₄, 90%),anhydrous tetrahydrofuran (THF) and dimethoxyethane (DME) were purchasedfrom Sigma-Aldrich. The various components were mixed to provide thespecified molar electrolyte solutions. Cyclic voltammetry testing wasconducted in a three-electrode cell with an Mg wire/ribbon asreference/counter electrodes. The electrochemical testing was conductedin an argon filled glove box with O₂ and H₂O amounts kept below 0.1 ppm.

Mg(BH₄)₂ in THF

Mg deposition and stripping was performed for Mg(BH₄)₂ in ethersolvents. FIG. 1a shows the cyclic voltammogram obtained for 0.5 MMg(BH₄)₂/THF where a reversible reduction/oxidation process took placewith onsets at −0.6 V/0.2 V and a 40% coulombic efficiency, as shown inFIG. 1a inset, indicating reversible Mg deposition and stripping. X-raydiffraction (XRD) of the deposited product following galvanostaticreduction from the above solution as shown in FIG. 1b denotes that thedeposited product is hexagonal Mg. The deposition of the hexagonalmagnesium demonstrates the compatibility of the electrolyte, Mg(BH₄)₂with Mg metal. The electrochemical oxidative stabilities measured onplatinum, stainless steel and glassy carbon electrodes were 1.7, 2.2 and2.3 V, respectively. These results denote that Mg(BH₄)₂ iselectrochemically active in THF such that ionic conduction andreversible magnesium deposition and stripping utilizing the electrolyteoccurs.

Mg(BH₄)₂ in DME

In addition to ether solvents, another solvent, dimethoxyethane (DME)having a higher boiling temperature than THF was utilized. The cyclicvoltammogram obtained for 0.1 M Mg(BH₄)₂/DME is shown in FIG. 1c . Ascan be seen in the Figure, there is an improvement in theelectrochemical performance compared to Mg(BH₄)₂/THF. As seen in theFigure, there is a 10 fold increase in the current density and areduction in the overpotential for deposition/stripping onsets at −0.34V/0.03 V vs. −0.6 V/0.2 V in THF). Additionally, the DME solvent basedelectrolyte demonstrated a higher coulombic efficiency at 67% incomparison to 40% in THF. These findings indicate the presence of Mgelectroactive species in higher concentration and mobility in DMEdespite the lower solubility of Mg(BH₄)₂ in DME versus THF.

IR and NMR spectroscopic analyses as shown in FIG. 2 were conducted for0.5 M Mg(BH₄)₂/THF and 0.1 M Mg(BH₄)₂/DME to characterize the magnesiumelectroactive species. The IR B—H stretching region (2000-2500 cm⁻¹)reveals two strong widely separated vibrations (Mg(BH₄)₂/THF: 2379 cm⁻¹,2176 cm⁻¹ and Mg(BH₄)₂/DME: 2372 cm⁻¹, 2175 cm⁻¹). The spectra for 0.1MDME and 0.5 M in THF are similar. The spectra are similar to covalentborohydrides and for Mg(BH₄)₂ solvates from THF and diethyl ether where2 hydrogen atoms in BH₄ ⁻ are bridge bonded to 1 metal atom (μ₂ ¹bonding). Therefore, we assigned the bands at the higher and lower B—Hfrequencies to terminal B—H_(t) and bridging B—H_(b) vibrations,respectively. The band and shoulder at 2304 and 2240 cm⁻¹ were assignedto asymmetric B—H_(t) and B—H_(b) vibrations, respectively. It issuggested that Mg(BH₄)₂ is present as the contact ion pair Mg(μ₂¹—H₂—BH₂)₂ which partially dissociates into Mg(μ₂ ¹—H₂—BH₂)⁺ and BH₄ ⁻as in reaction (1). In one aspect, since the B—H peaks are likelyoverlapping, it is not possible to distinguish all the species.

Mg(μ₂ ¹—H₂—BH₂)₂⇄Mg(μ₂ ¹—H₂—BH₂)⁺+BH₄ ⁻  (1)

Where Mg(μ₂ ¹—H₂—BH₂)⁺ may further dissociate:

Mg(μ₂ ¹—H₂—BH₂)⁺⇄Mg²⁺+BH₄ ⁻  (2)

For Mg(BH₄)₂/DME, while the main features present in THF were retained,νB—H_(t) broadening and shifting to lower values accompanied withrelative weakening of νB—H_(b) intensity was observed. While νB—H_(t)broadening suggests a more pronounced presence of some species relativeto that in THF, the band maximum shift indicates a more ionic B—H bond.The νB—H_(t) shift is similar to those resulting from an enhanced BH₄ ⁻iconicity, such as in stabilized covalent borohydrides. In addition, therelative weakening in νB—H_(b) intensity suggests a more pronouncedpresence of free BH₄ ⁻ anion. The NMR results for BH₄ ⁻ in DME, as shownin FIGS. 2b and 2c , display an increased boron shielding by about 0.5ppm as denoted by the center position of quintet in ¹¹B NMR and slightlyreduced proton shielding by about 0.01 ppm, as denoted by the quartet in¹H NMR consistent with a higher B—H bond ionicity compared to that inTHF. These results are evidence of weaker interactions between Mg²⁺ andBH₄ ⁻ within the ion pair and an enhanced dissociation in DME perreactions (1) and (2). So despite the fact that DME has a slightly lowerdielectric constant (7.2) compared to THF (7.4), its chelationproperties due to the presence of two oxygen sites per molecule resultedin an enhanced dissociation and thus an improved electrochemicalperformance.

Mg(BH₄)₂ and LiBH₄ in DME

As recited above, the electrolyte may include an acidic cation additive.In one aspect, the acidic cation additive may include the followingcharacteristics: 1) a reductive stability comparable to Mg(BH₄)₂, 2)non-reactive, 3) halide free and 4) soluble in DME. One such materialthat includes these properties is LiBH₄. Mg deposition and stripping wasperformed in DME using various molar ratios of LiBH₄ to Mg(BH₄)₂. Asshown in FIG. 3a cyclic voltammetry data was obtained for 3.3:1 molarLiBH₄ to Mg(BH₄)₂. Including the LiBH₄ material in the electrolyteincreased by 2 orders of magnitude the current density as denoted by theoxidation peak current Jp=26 mA cm⁻². Additionally, the electrolyte hada higher coulombic efficiency of up to 94%.

Referring to FIG. 3b , the deposition and stripping currents aredisplayed for magnesium based on the absence of Li followinggalvanostatic deposition and also the lack of electrochemical activityin LiBH₄/DME solution. Enhanced BH₄ ⁻ ionicity as shown in FIG. 3cdisplayed as lower νB—H_(t) and higher νB—H_(b) values were obtained.The enhanced properties indicate that the acidic cation additiveincreases Mg(BH₄)₂ dissociation as indicated by the B—H bands forLiBH₄/DME which occur at lower values.

Magnesium Battery with Mg Anode, Chevrel Phase Cathode and Mg(BH₄)₂ andLiBH₄ in DME

A magnesium battery was tested using an electrolyte for 3.3:1 molarLiBH₄ to Mg(BH₄)₂. The cathode of the test battery included a cathodeactive material having a Chevrel phase Mo₆S₈. The anode for the testbattery included an Mg metal anode. Referring to FIG. 4, the testbattery demonstrated reversible cycling capabilities at a 128.8 mA g⁻¹rate. As can be seen in the Figure, the charge and discharge curvesindicate reversible cycling of a magnesium ion.

Synthesis of MgB_(x)H_(y), where x=11-12 and y=11-12

A mixture of 5.0 g (0.0409 mol) decaborane (B₁₀H₁₄) and 2.43 g (0.0450mol, 1.1 eq.) magnesium borohydride (Mg(BH₄)₂) is prepared in a 100 mlSchlenk flask inside an argon filled glovebox. The flask is transferredfrom the glovebox to a nitrogen Schlenk-line and fitted with a refluxcondenser. To this is added 50 ml Diglyme (C₆H₁₄O₃) via cannulatransfer. Upon solvent addition, vigorous gas evolution begins, and ayellow homogeneous solution is formed. When gas evolution has ceased,the mixture is slowly heated to reflux using a silicon oil bath. As thetemperature of the mixture increases, vigorous gas evolution beginsagain, and is maintained for approximately one hour. The mixture is heldat reflux for 5 days before being allowed to cool to room temperature.Following cooling, the solvent is removed under vacuum to give a paleyellow solid. The crude product obtained at this stage may be purifiedby dissolving in a minimal amount of hot (120 C) DMF. The resultingsolution is allowed to cool to room temperature, and a colorlessprecipitate is observed which is isolated by filtration.

The product obtained as outlined above was analyzed using an NMR scan.As can be seen in FIG. 5, the NMR results confirm the successfulsynthesis of MgB₁₂H₁₂. As can be seen in the Figure, the product assynthesized shows the 11B Nuclear Magnetic Resonance of B₁₁H₁₁ andB₁₂H₁₂ in both the crude product and in the filtered product.

The product as synthesized was subjected to electrochemical testing. Theelectrochemical testing procedure included cyclic voltammetry collectedusing a 3-electrode cell in which the working electrode was platinum andboth the counter and reference electrodes were magnesium. A plot of theelectrochemical testing data is shown in FIG. 6 as a plot of the currentdensity as a function of the Potential. As can be seen in FIG. 6, thesynthesized product is stable against both electrochemical reduction(>−2 V vs. Mg) and oxidation (>3 V vs. Mg). The synthesized compoundwill allow a magnesium battery utilizing the synthesized compound as anelectrolyte to operate at a high voltage necessary to achieve sufficientenergy density for use in numerous applications such as in automotiveapplications.

The invention is not restricted to the illustrative examples describedabove. Examples described are not intended to limit the scope of theinvention. Changes therein, other combinations of elements, and otheruses will occur to those skilled in the art.

1. An anhydrous electrolyte for a magnesium battery comprising: amagnesium salt having the formula: Mg(BH₄)₂; and a solvent, themagnesium salt dissociating in said solvent to Mg electroactive speciesBH₄ ⁻ and Mg²⁺.
 2. The anhydrous electrolyte of claim 1 wherein thesolvent is an aprotic solvent.
 3. The anhydrous electrolyte of claim 2wherein the aprotic solvent is selected from: tetrahydrofuran (THF) anddimethoxyethane (DME).
 4. The anhydrous electrolyte of claim 1 whereinthe solvent is an ionic liquid.
 5. The anhydrous electrolyte of claim 1wherein the magnesium salt has a molarity of from 0.01 to 4 molar. 6.The anhydrous electrolyte of claim 1 further including a chelating agentselected from monoglyme, diglyme, tetraglyme and crown ethers.
 7. Theanhydrous electrolyte of claim 1 further including an acidic cationadditive selected from: lithium borohydride, sodium borohydride andpotassium borohydride.
 8. A magnesium battery comprising: a magnesiummetal containing anode; an anhydrous electrolyte including a magnesiumsalt having the formula: Mg(BH₄)₂; and a solvent, the magnesium saltdissociating in said solvent to Mg electroactive species BH₄ ⁻ and Mg²⁺;a cathode; and wherein magnesium cations are reversibly stripped anddeposited between the anode and cathode.
 9. The magnesium battery ofclaim 8 wherein the solvent is an aprotic solvent.
 10. The magnesiumbattery of claim 9 wherein the aprotic solvent is selected from:tetrahydrofuran (THF) and dimethoxyethane (DME).
 11. The magnesiumbattery of claim 8 wherein the solvent is an ionic liquid.
 12. Themagnesium battery of claim 8 wherein the magnesium salt has a molarityof from 0.01 to 4 molar.
 13. The magnesium battery of claim 8 furtherincluding a chelating agent selected from monoglyme, diglyme, tetraglymeand crown ethers.
 14. The magnesium battery of claim 8 further includingan acidic cation additive selected from: lithium borohydride, sodiumborohydride and potassium borohydride.